The present invention relates to a non-sintered type electrode for use in storage batteries (secondary batteries), such as alkaline storage battery, lithium ion storage battery and polymer lithium storage battery.
Various types of electrodes are known for the storage battery (secondary battery). Non-sintered electrodes are defined to be the electrodes that can be obtained without sintering and are fabricated by coating or pressing an active material for battery or active material retaining medium onto an electrode support.
The process of fabricating a nickel positive electrode of alkaline storage battery as one example of sintered electrodes is described below. A microporous sintered plaque obtained by sintering nickel powder is impregnated with an aqueous solution of nickel nitrate or the like thereby to add nickel salt, and, after drying, the sintered plaque is immersed in caustic alkali aqueous solution to convert the nickel salt to nickel hydroxide. This method has the disadvantage that the process is complicated and the filling density of nickel hydroxide as an active material is reduced in comparison with the non-sintered electrode described later. In spite of this disadvantage, this electrode has an excellent high-rate discharge characteristic and a long cycle life, and finds wide application in a variety of field.
A method called pocket system was previously used for the manufacture of non-sintered electrode, while paste applying or pressing methods have become the mainstream in the recent years. In the paste applying method, the active material for battery itself or active material retaining medium is prepared into a paste with water or an organic solution, this paste is applied on an electrode support and dried. In the pressing method, on the other hand, the active material for battery or active material retaining medium in the form of powder directly fills the electrode support by being pressed thereonto with a press machine or the like.
A variety of materials are used for these non-sintered electrodes, including metal foil, perforated metal plate, metal net, expanded metal, foamed porous metal material and the like. These materials have been applied with their properties and forms varied-accordingly so as to suit each battery system.
For example, foamed nickel porous material having a three dimensionally reticulated structure has been commonly used as the electrode support for the positive electrode of non-sintered type used in alkaline storage batteries such as nickel-hydrogen storage battery or nickel-cadmium storage battery. For the negative electrode, on the other hand, punched metal has been mainly employed.
The method using foamed nickel porous material is a simple method of electrode production. Further, the availability of a foamed nickel porous material of high porosity makes it possible to fill it with nickel hydroxide to a high density and therefore a high-capacity battery can be produced. The foamed nickel porous material, however, needs to be produced by electroplating and therefore has the disadvantage of high material cost.
In view of this, a non-sintered electrode is under development using a low-cost punched metal or expanded metal in place of the foamed nickel porous material as an electrode support. These electrode supports have no three-dimensional structure unlike the sintered plaque or the foamed-nickel porous material. As a result, an electrode made of these electrode supports has a low ability to hold an active material and the active material is liable to fall off during electrode fabrication or repeated charging and discharging. Further, due to the low electronic conductivity in the electrode thickness direction and a poor electrode characteristic, which is a serious hindrance to the application in the nickel positive electrode of alkaline storage battery, these electrode supports find no practical applications except for special types of electrodes.
Japanese Laid-Open Patent Application No.7-130370 and No.9-7603 disclose technologies for improvement of these electrode supports. The electrode support according to JP Laid-Open No.7-130370 is constructed of flat metal sheet or flat metal foil and thus is weak in the adhesion between the active material layer and electrode support. Separation of active material from electrode support occurs particularly in the application as the electrode of a storage battery due to changes in volume of the active material caused by repeated charging and discharging. Current collecting ability decreases accordingly, as a result of which the battery characteristics are deteriorated.
As a countermeasure for this drawback, formation of minute irregularities using metal powder is proposed in JP Laid-Open No.9-7603. The adhesion between the active material layer and the electrode support is thereby improved. However, the production cost of electrode supports will be raised because of sintering in an inactive gas atmosphere or electroplating methods required for forming the minute irregularities layer.
In both of the above electrode supports, furthermore, in the case of corrugating the electrode supports so that they have a three-dimensional structure, they are more subjected to deformation and elongation during the compressing process for filling the active material at a high density. As a result, cracks or rupture occur in the electrode support, which leads to troubles such as decrease in current collecting ability of the electrode and micro-short circuit when assembled as a battery. Moreover, the above mentioned deformation and elongation of the electrode support also set a limit to the high-density filling and a battery of large discharge capacity cannot be obtained.
Meanwhile, efforts have been made to improve the electrode characteristics such as retaining property of active material and electronic conductivity for the negative electrode of alkaline storage battery (cadmium electrode or hydrogen-absorption alloy electrode), using inexpensive punched metal or expanded metal while exploiting their advantages. Further improvement is desired to achieve a more efficient high-rate discharge characteristic and a longer cycle life, which are still unsatisfactory in these negative electrodes.
These demands are also applicable to other types of batteries such as for example lithium ion storage battery or polymer lithium storage battery. There has generally been a desire for an electrode using a low cost electrode support while exhibiting excellent performance.
The above-mentioned method of electrode production using a punched metal or expanded metal as an-electrode support has the advantage that a powder of active material made into a paste with a solution of a high polymer binder and a conductive powder is coated and dried on the electrode support and thus the electrode can be easily produced. The adhesion between the metal substrate acting as the electrode support and the active material layer is generally weak so that the active material is liable to peel off from the metal substrate in an application using the electrode for batteries. In the case where the electrode support acts as a current collector, the electrical resistance of the electrode increases thereby causing a reduced discharge voltage and discharge capacity. In order to solve this problem, adding a great amount of binder to the active material layer suppresses the separation. The resultant reduced reactivity of the active material, however, has an adverse effect on the discharge characteristic.
In a method for strengthening the adhesion between the electrode support and the active material layer, a thermoplastic resin layer functioning as a binder is formed on the surface of the electrode support. Then, the active material is coated on the thermoplastic resin layer and the electrode is heated, to improve the adhesion between the electrode support and the active material layer. This method, however, has a disadvantage that a resin insulating layer is formed between the metal electrode support and the active material layer with the result that the current collecting characteristic of the electrode is reduced, thereby reducing the reactivity of the electrode.
As described above, these problems are difficult to solve when a comparatively flat metal substrate-is used as an electrode support.
Accordingly, an object of the present invention is to provide all improvement in a non-sintered type electrode with an active material or active material retaining medium coated or pressed on an electrode support, in order to achieve an improved adhesion and improved electronic conductivity between the active material layer and the electrode support, while maintaining the advantage of low material cost.
Another object of the present invention is to provide an improved non-sintered electrode which is favorably used as the nickel electrode of alkaline storage batteries such as nickel hydrogen storage battery and nickel-cadmium storage battery, the hydrogen-absorption alloy electrode which uses hydrogen-absorption alloy powder, and the cadmium electrode, as well as for the electrodes of lithium ion storage battery and polymer lithium storage battery.
In order to achieve the above objects, the present invention provides a non-sintered type electrode comprising an electrode support made of a metal sheet having minute surface irregularities on which is coated or pressed an active material for battery or an active material retaining medium, characterized in that said surface irregularities are formed by a mechanical method such that protrusions and indentations are configured with a center-to-center pitch in the range of from 50 to 300 xcexcm, and such that the apparent thickness after forming the surface irregularities is at least three times as large as the thickness before the formation of the surface irregularities.
The protrusions and indentations should preferably be formed in a substantially tapered shape such as conical shape, but may also be formed in hemispheric shape.
The apparent thickness of the electrode support after forming the surface irregularities should preferably be 200 xcexcm or more and in particular 400 xcexcm or more, and should be at least five times as large as the thickness before the formation of the surface irregularities.
The electrode support may be constructed of a punched metal or a non-punched metal sheet, of which material thickness before the formation of the surface irregularities should preferably be in the range of from 10 to 80 xcexcm and in particular from 20 to 60 xcexcm. The electrode support should preferably be made of nickel sheet, but may be also constructed of steel sheet or nickel-plated steel sheet.
The center-to-center pitch of the protrusions and indentations should preferably be in the range of from 50 to 300 xcexcm, and in particular 100 to 200 xcexcm.
In addition to the above construction, it is preferable that the metal sheet with the surface irregularities has innumerable minute holes formed by piercing through the tops of the protrusions. In particular, it is preferable to construct the electrode support by forming such minute holes in a non-punched metal sheet.
The protrusions and indentations of the surface irregularities should preferably be formed either at random or in order with the respective numbers of protrusions and indentations in ranges of from 80 to 20% and from 20 to 80% per unit area. The arrangement of the protrusions and indentations may be such that they are formed alternately in one direction or in both longitudinal and transverse directions.
Owing to the above construction, the electrode support according to the present invention has an advantageous feature of low manufacturing cost, since it can be fabricated simply by a mechanical method just like common punched metal which is obtained from a nickel-plated steel sheet by a mechanical process. In addition to this, the electrode support of the present invention is formed with minute surface irregularities with the center-to-center pitch between adjacent protrusions and indentations in the range of from 50 to 300 xcexcm, whereby the apparent thickness of the electrode support is increased to at least three times as large as the unprocessed material thickness. Thanks to this drastic transformation of the structure into three-dimensional form, the active material retaining ability of the electrode support is remarkably improved owing to the minute surface irregularities, and separation of the active material layer from the electrode support is suppressed in comparison with a two-dimensional electrode support such as punched metal. The electronic conductivity in the electrode thickness direction is also enhanced. The utilization rate of active material is accordingly increased, whereby the battery of higher capacity can be obtained.
There might have been a trouble that such machining process causes the tensile strength of the electrode support to decrease and that the yield of product is reduced accordingly, but this problem was avoided by providing a strip-form solid portion in the electrode support where no punched holes or irregularities are provided.
The electrode support according to the present invention as described above can constitute a non-sintered electrode of excellent quality. Specifically, the above technologies can be applied for the fabrication of a nickel positive electrode of an alkaline storage battery. In that case, the active material should preferably comprise nickel hydroxide powder of which surface is coated with cobalt oxide of higher order at the rate of 2 to 10 wt % in relation to 100 wt % of nickel hydroxide, or alternatively, cobalt oxide of higher order or nickel may be mixed in the nickel hydroxide powder. In the case where the cobalt oxide of higher order is solely used, it should be contained at the rate of 2 to 10 wt % in relation to 100 wt % of nickel hydroxide, whereas if nickel is solely used, it should be contained at the rate of 10 to 25 wt % in relation to 100 wt % of nickel hydroxide. A nickel positive electrode of extremely high utilization rate can be thereby obtained.
Furthermore, the above technologies can be also applied for the fabrication of a hydrogen-absorption alloy negative electrode of an alkaline storage battery. In that case, the active material retaining medium may comprise hydrogen-absorption alloy powder alone, or a conductive agent selected from nickel, copper, and carbon, may be contained therein at the rate of 0.5 to 10 wt % in relation to 100 wt % of hydrogen-absorption alloy powder. Such nickel or copper may be coated on the surface of the hydrogen-absorption alloy powder in the same amount as mentioned above. A hydrogen-absorption alloy negative electrode of extremely high utilization rate and excellent high-rate discharge characteristic can be thereby obtained.
It is also possible to apply the electrode support of the present invention to a lithium ion storage battery or lithium polymer storage battery, and to improve the electrode characteristics thereof by selecting the materials appropriately.
The present invention also provides, in order to achieve the above said objects, a method of manufacturing a non-sintered type electrode characterized in that minute irregularities are formed on the surface of a metal sheet by press-machining using metal molds such that protrusions and indentations are configured with a center-to-center pitch in the range of from 50 to 300 xcexcm and such that the apparent thickness after forming the surface irregularities is at least three times as large as the thickness before the formation of the surface irregularities, thereby fabricating an electrode support, and an active material for battery or an active material retaining medium is coated or pressed on the surface of the thus obtained electrode support.
In the above process of press-machining the metal sheet with metal molds, minute holes should preferably be formed simultaneously with the formation of the surface irregularities by piercing through the tops of the protrusions. Also, it is preferable that each of the protrusions and indentations is formed in a substantially tapered shape.